Silica sol-masking in galvanizing process



United States Patent Chemical Company, Chicago, 111., a corporation of Delaware No Drawing. Filed July 27, 1960, Scr. No. 45,545 4 Claims. (Cl. 117-5.5)

The present invention relates to an improved method for galvanizing ferrous metals. Specifically, the invention relates to an improved system for single side galvanizing of various types of ferrous metals.

Galvanizing is a well known method for coating the surfaces of ferrous metals to improve their ability to Withstand various types of corrosion. In addition to improving the corrosion resistance of ferrous metals, galvanizing quite frequently improves the outer appearance of the metal piece so coated. While galvanizing is acceptable for forming protective coatings on a large number of ferrous metal parts, in some applications, the surface formed by the galvanizing operation is undesirable. This is particularly true where it is necessary to weld pieces of metal together since it has been experienced that the zinc film on ferrous metals will tend to form a harmful coating around the electrodes when resistance welding techniques are employed.

In the production of automobile bodies, it is desirable that certain surfaces be protected by galvanizing while, in other instances, it is beneficial that a metal surface does not have a galvanized coating, thereby rendering resistance welding and surface finishing operations, such as painting or lacquering, more simple to perform.

To overcome the difficulties experienced where it is sought to protect only a portion of a metal surface with a galvanized coating, there has arisen a process which is generally known to the art as single side galvanizing.

When it is desired to prevent galvanizing from adhering to certain surfaces on a ferrous metal part that is to be galvanized, it has been found that such surfaces may be treated with certain chemicals which prevent the adherence of the galvanized coating thereto. When the metal part is dipped into the hot galvanizing bath of molten zinc, these chemicals tend to form a protective barrier which effectively acts to prevent the zinc from adhering to the coated portion.

Several advances have been made in the art of single side galvanizing with respect to the chemicals employed. Use has been made of alkali metal aluminates, particularly sodium aluminate, and sodium meta silicate for such purposes. While these have proven satisfactory in many respects, in the single side galvanizing of sheet steel, it has been the experience of the art that such materials have a serious disadvantage, since they are oftentimes difficult to remove. This problem is attributed to the fact that these chemicals must be applied as thick coatings in order to be operative.

In order that the zinc galvanized coatings properly adhere to ferrous metal parts, it is essential that the part be completely free of dirt, oils, iron salts, and other undesirable impurities. In order to prepare metal surfaces for galvanizing there are now in use two basic type systems for preparing the metal surfaces prior to the actual galvanizing. In the first system, it is customary to clean the surface after cold rolling using conventional chemical cleaning operations such as vapor degreasing, chlorinated solvent degreasing, or hot alkaline aqueous degreasing type solutions.

The other system used for cleaning ferrous metal surfaces prior to galvanizing is to subject the metal parts to elevated temperatures, under oxidizing conditions wherein 3,177,285 Patented Apr. 6, 1965 all surface residue from cold rolling operations is removed by combustion. The heating step forms oxides on the surface of the metal which are then removed in the second step of the operation which consists in placing the metal parts in a second furnace at annealing temperatures which are at least 1250 F., with the interior of the furnace being blanketed by a reducing gas. After this, the ferrous metal surface is acid pickled, rinsed and air dried.

An object of this invention is to provide an improved method for treating only a portion or one side of a ferrous metal surface to prevent the adherence thereto of zinc in the hot dip galvanizing of such surfaces. Another object is to provide a coating which may be readily removed, after galvanizing.

A further object is to provide a thin coating which may be economically applied to the surface of the ferrous metal prior to galvanizing, whereby such surfaces tend to be non-adherent to hot zinc coatings. Other objects will appear hereinafter.

In accordance with this invention, it has been found that it is possible to selectively galvanize surfaces of metal parts without galvanizing other surfaces by placing upon the surfaces not desired to be galvanized, a coating of colloidal silica, and then galvanizing the parts so treated. The colloidal silicas used in the practice of the invention are preferably colloidal silica sols which have as a continuous phase, a liquid; and as a dispersed phase, a finely divided amorphous silica which has an average particle size of less than one micron and usually less than millimicrons.

In a preferred practice of the invention, the colloidal silica applied to the surfaces of the metal parts not to be galvanized, are aqueous colloidal silica sols. Colloidal silica sols were well known materials and are commercially available from several sources of supply. A typical group of commercially available silica sols that may be used in the practices of the invention are those silica sols sold under the trade name, Nalcoag. Typical sols of this type are presented below in Table I.

TABLE I Silica Sol I II III IV V Percent colloidal silica as SiOg 15 30 35-36 21-22 49-50 p151. 8.6 10. 2 8.6 3. 7 9.0 Viscosity at 77 F.,

cps Less Less Less Less 20-30 than 5 than 5 than 5 than 10 Specific gravity at 68 F 1.09 1. 205 1. 255 1. 06 1. 385 Average surface area ml per gram of S101 330430 -270 135-190 135-190 120-150 Average particle size mlllimlcrons- 7-9 11-16 16-22 16-22 20-25 Density, #/gallon at 62 F 9.1 10.0 10.5 8.8 11.6 Freezing point, F 32 32 32 -10 32 N320, percent 0. 04 O. 40 0. 10 0. 05 0.30

Other silica sols that may be used in addition to those mentioned above may be prepared by using several well known conventional techniques. Perhaps the most convenient method ofmaking aqueous colloidal silica sols is described in Bird, US. 2,244,325, wherein a dilute solution of an alkali metal silicate is passed in contact with a cation exchange resin in the hydrogen form whereby the silicate is converted to a dilute aqueous colloidal silica sol. Silica sols of the type described by Bird as produced are relatively dilute. They may be concentrated to more usable concentrations from the standpoint of shipping and application, by employing the techniques described in either Bechtold et al., US. 2,574,902; Bragg et al., US. 2,680,721; or Parma et al., US. 2,601,235. Another o type of silica sol which may While aqueous colloidal silica sols are preferred for reasons which will be more fully explained hereinafter,

races be used in the practice of the invention is described in the specification of Router, U59

it will be understood that other forms of colloidal silica 7 which are prepared by dispersing or suspending'various types of finely divided amorphous silica powders into various types of liquid media, e.g., water, poar organic solvents, or non-polar organic solven s. Examples of such dispersions of finely divided silica materials are described in the specification of Nohejl, US. 2,787,967.

A further-highly practical variant of the above described silica sols is prepared by the addition thereto of appropriate thickening agents which are useful in formu lating a sol with viscosity specifications calculated for efficient use with any given particular spray or roller coat-' ing system. Typical thickeners which have been added to silica sols for this purpose are carboxymethylcellulose compounds prepared in gran ar powder form from the productof the substitution reaction of an alkali-treated cellulose with sodium monochloroacetate. 'By this process, a variety of cellulosic compounds can be prepared, having physical characteristics determined by the chain lengt. of the original cellulose starting material, as well as the degree of substitution during, the above outlined reaction.

varying chain lengths and degrees of substitution. The invention includes, however, the use of any other commercially available thiclicners lmown to be useful in aqueous and/or non-aqueous systems.

Since galvanizing is a relatively inexpensive method of protecting various types of ferrous metals against corrosion as well as to improve their appearance, it is important that any chemical additive used for purposes of preventing such coatings from adhering to certain portions of the metals treated, does not materially add to the cost of such operations. The colloidal silicas described above, particularly the aoueous colloidal silica sols, in addition to being commercially available, are relatively inexpensive. When used in trchting surfaces to prevent. the adherence of zinc coatings, it has been discovered that the aqueous colloidal silicalsols, as well as other finely divided colloidal silica materials may be used at relatively low concentrations with excellent results being achieved.

In most instances it is expedient to apply the finely divided colloidal silica from a liquid media by either coat.

ing or spraying. In the case of aqueous media, it has Additional choicev of thickens-rs may be 'maue from among methylcellulose compounds having been found that these materials may be diluted with' waterto produce concentrations. containing aslittle as 1% by weight of silica expressed as SiO As a general rule,

however, it has been found that the concentration of the silica sols and colloidal silica materials when applied as.

a liquid can be maintained within the range of from about 1% to 30% by weight with the best results being at higher concentrations to obtain high density films.

The colloidal silicas of the invention are relatively free of alkali metal oxides and have large. specific surface areas. Asia. general rule, the SiO to Na O ratio will vary from as little as 102 1 to between 500:1. In some instances, where thesilica sols have been deionized, it is possible for these colloidal silica materials tobe completely free from metal ions. Similarly the surface areas while capable of variations between 25 sq. meters per gram to about 1000 sq. meters per gram, usually 'havesurface areas within the range of about 25 400 sq. meters per gram.

meters per grain to,

in using the'aqueous colloidal silica sols of the invention,.improvcd adhesion to the metal surface is obtained by adjusting the pH on the acid side between 3-6' and using certain we'tlng agents to produce film uniformity. It is desirable that the films of colloidal silica be drie prior to the galvanizing operation.- The drying operation may be conducted over various'periods of time and at a variety of temperatures. When the treatedparts are subiected to continuous high speed galvanizing operations,

relatively elevated temperatures, e.g. in'excess of 225 F,

are beneficially employed to remove substantially all the Water from the aqueous silica sols in order to provide a tenacious'thin film of colloidal silica on the surface of the metal prior to'annealing and galvanizing. i I

ltis sometimes'desirableto remove any. silica that might re; ain on the. surfaces after the, piece has been galvanized. 'R'emcval may be accomplishedby standard polishing methods.

Example. I

In this example, the test. situs comprised a large galvanizing line located in' 'a steel mill. The metal was cleaned prior to the galvanizing step. This particular mill subjected the stack to an annealing process prior to galvanizing. Before annealing, there vwas ir1- stalled an oven which heated the surface temperature of the metal up to about 350 F. Prior to the steel coil being drawn tlircughthe-oven, a bath was installed which contained a dilute solution of the aqueous colloidal silica sol, corresponding to H, Table I. The sol had been diluted with tap Water to produce .a finished silica con: centration of 15% by'weightandwas adjustedto a pH of 3.5 witna Wetting agent addedat a concentration of 0.5%. Positioned directly above the bath was a moving roll which picked up the colloidal silica and deposited it. on the topside of the surface of the steel coil. steel coil was then passed through the drying ovenand then annealed and galvanized in a standard manner. The zinc coating did not'adhere to'the side .on which the silica coating'was applied.

An important advantage of the invention is that even though they are subjected to relatively high temperatures in both the drying and galvanizing operations, they are substantially free from metal ions Which, in some instances, will tend to react with the metal surface thereby interfering with its surface properties and characteristics.

It is believed that one of the reasons why the colloidal silica sols described are so' effective in preventing galvanized coatings fromadhering to the surfaces of the. metal treated therewith, is. that they tend to distribute.

very fine, particles of colloidal silica uniformly over the surface of the metal. This distribution of extremely fine, submioroscopic particles. not only tends to prevent the adherenceiof the zinc coating to the treated surface, but also tends to improve the surface, characteristics of the metal treated.

While the. various'forms ofcolloidal silica are easily applied to metal surfaces for the PillPOStiSmGf preventing the adherence ofwgalvanizing thereto, it has been. found that the surfacesxmay be more readily Wet bythe various. colloidal silicas, particularly. the aqueous colloidalsilica sols, if they are combined with minor amounts of compatible wettingagents." In treating baths. and sprays used to coat the surface 'of the metal prior to galvanizing, amounts of surface active agentsranging from as little as 0.1 to as much as 2 by weight, may be used to improve the wetting ability of the. colloidal silicas. The preferred group of wetting agents are the non-ionic type which are exemplified, by octylphenol which has been reacted with nine mole of ethylene oxide. Similarly, the block polymers sold under the trade name of Pluronics and whio i are the subject-matter of Lunsted, US. Patent No. 2,674,619, are also admirably suited for use in conjunction with colloidal silicas.

The.

thefine silica coatings of.

The invention is hereby claimed as follows:

1. A method for galvanizing only a portion of a forrous metal object which comprises the steps of applying to the surfaces of the object not to be galvanized, a coating of an aqueous colloidal silica sol, consisting essentially of silica particles of a size less than 150 millimicrons and in an amount of from 1% to 30% by weight SiO heating the surface so treated to a temperature sufiicient to substantially dry the sol, and form a thin film of colloidal silica, and galvanizing the meta-l object.

2. A method for galvanizing only a portion of a ferrous metal object which comprises the steps of applying to the surfaces of the object not to be galvanized, a coating of an aqueous colloidal silica sol, consisting essentially of silica particles of a size less than 15 millimicrons and in an amount of from 1% to 30% by weight SiO heating the surface so treated to a temperature suflicient to substantially dry the sol, and form a thin film of colloidal silica, and galvanizing the metal object, and then removing any excess silica from the non-galvanized surfaces.

3. A method for galvanizing only a portion of a ferrous metal object which comprises the steps of cleaning the surfaces of the metal to remove dirt and oxide coatings therefrom, applying to the surfaces of the object not to be galvanized, a coating of an aqueous colloidal silica sol, consisting essentially of silica particles of a size less than 150 millimicrons and in an amount of from 1% to 30% by Weight SiO heating the surface so treated to a temperature sufficient to substantially dry the sol, and form a thin film of colloidal silica and galvanizing the metal object.

4. A method for galvanizing only a portion of a ferrous metal object which comprises the steps of cleaning.

the surfaces of the metal to remove dirt and oxide coatings therefrom, applying to the surfaces of the object not to be galvanized, a coating of an aqueous colloidal silica sol, consisting essentially of silica particles of a size less than millimiorons and in an amount of from 1% to 30% by weight SiO heating the surface so treated to a temperature sufiicient to substantially dry the so], and form a thin film of colloidal silica, and galvanizing the metal object and then removing any excess silica from the non-galvanized surfaces.

References Cited by the Examiner UNITED STATES PATENTS RICHARD D. NEVIUS, Primary Examiner. JOSEPH B. SPENCER, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,177,085 April 6, 1965 Nicholas M. Adams It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 30, for "11" read III line 73, for "Lunsted" read Lundsted column 6,- line 26, for "117-515" read 117-5 .5

Signed and sealed this 21st day of September 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A METHOD FOR GALVANIZING ONLY A PORTION OF A FERROUS METAL OBJECT WHICH COMPRISES THE STEPS OF APPLYING TO THE SURFACES OF THE OBJECT NOT TO BE GALVANIZED, A COATING OF AN AQUEOUS COLLOIDAL SILICA SOL, CONSISTING ESSENTIALLY OF SILICA PARTICLES OF A SIZE LESS THAN 150 MILLIMICRONS AND IN AN AMOUNT OF FROM 1% TO 30% BY WEIGHT SIO2, HEATING THE SURFACE SO TREATED TO A TEMPERATURE SUFFICIENT TO SUBSTANTIALLY DRY THE SOL, AND FORM A THIN FILM OF COLLOIDAL SILICA, AND GALVANIZING THE METAL OBJECT. 